Electronic components of antilock brake systems

This small control computer is normally mounted inside the trunk on the wheel housing, mounted to the master cylinder, or is part of the hydraulic control unit. It monitors system operation and controls antilock function when needed. The module relies on inputs from the wheel speed sensors and feedback from the hydraulic unit to determine if the antilock brake system is operating correctly and to determine when the antilock mode is required.

Brake pedal sensor:

The antilock brake pedal sensor switch is normally closed. When the brake pedal travel exceeds the antilock brake pedal sensor switch setting during an antilock stop, the antilock brake control module senses that the antilock brake pedal sensor switch is open and grounds the pump motor relay coil. This energizes the relay and turns the pump motor on. When the pump motor is running, the hydraulic reservoir is filled with high-pressure brake fluid, and the brake fluid will be pushed up until the antilock brake pedal switch closes. When the antilock brake pedal sensor switch closes, the pump motor is turned off and the brake pedal will drop some with each ABS control cycle until the antilock brake pedal sensor switch opens and the pump motor is turned on again. This minimizes pedal feedback during ABS cycling.

Indicator lights:

Most ABS-equipped vehicles are fitted with two different brake warning lights. One of the warning lights is tied directly to the ABS, whereas the other lamp is part of the base brake system. All vehicles have a red warning light. This lamp lights when the brake fluid level is low, when there is a problem with the brake system, or when the parking brake is on. An amber warning lamp lights when there is a fault in the ABS. Both lamps will illuminate if there is a major problem in the base system, causing the ABS to be inhibited.

Lateral acceleration sensor:

Used on some vehicles with stability control, this switch monitors the sideward movement of the vehicle while it is turning a corner. This information is sent to the control module to ensure proper braking during turns.

The pressure differential switch is located in the modulator unit. This switch sends a signal to the control module whenever there is an undesirable difference in hydraulic pressures within the brake system.

Relays:

Relays are electromagnetic devices used to control a high-current circuit with a low-current switching circuit. In ABS, relays are used to switch motors and solenoids. A low-current signal from the control module energizes the relays that complete the electrical circuit for the motor or solenoid.

Toothed ring:

The toothed ring, also called a toner ring, can be located on an axel shaft, differential gear, or a wheel’s hub. This ring is used in conjunction with the wheel speed sensor. The ring has a number of teeth around its circumference. The number of teeth varies by manufacturer and vehicle model. As the ring rotates and each tooth passes by the wheel speed sensor, an AC voltage signal is generated between the sensor and the tooth. As the tooth moves away from the sensor, the signal is broken until the next tooth comes close to the sensor. The end result is a pulsing signal that is sent to the control module. The control module translates the signal into wheel speed. The toothed ring may also be called the reluctor, tone ring, or gear pulser.

Wheel-speed sensor:

The wheel-speed sensors are mounted near the different toothed rings. As the ring’s teeth rotate past the sensor, an AC voltage is generated. As the teeth move away from the sensor, the signal is broken until the next tooth comes close to the sensor. The end result is a pulsing signal that is sent to the control module. The control module translates the signal into wheel speed. The sensor is normally a small coil of wire with a permanent magnet in its centre.